You'd be surprised how many people walk around with terrible vision, without knowing it. My guess is that computer-related jobs require good vision, so people in these jobs are more likely to discover and treat their vision problems.
Also, until relatively recently glasses were uncool and unfashionable. Computer geeks generally pay less attention to such norms and choose function over form, so a geek with marginally poor eyesight would perhaps be more likely to go for glasses than a non-geek, who would survive without for appearance purposes.
I think jobs with computers are, in many ways, more forgiving to those with bad eyesight than other jobs. I have bad vision (just enough so I can't drive) but thanks to Gtk / Compiz, I can arbitrarily change the font size to be as big as I like it, and Orca and other screenreaders would help if I were completely blind. However, if I were a scientist or a taxicab driver or at a factory or throwing darts for a living, I'd be much worse off because I couldn't make those accommodations.
